Azure Virtual Machine - Availability & Scale

1. Availability set

Availability sets are logical groupings of VMs that reduce the chance of correlated failures bringing down related VMs at the same time.

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You can configure the availability set and availability zone only during VM creation. Once a VM is placed in an availability set or availability zone, there is no way to remove/update the VM's availability settings. You have to delete and recreate the VM with the updated configuration.

When you create a VM in an availability set, you place the VM in a fault domain and an update domain:

A fault domain is a physical grouping of servers in a server rack. You can configure up to 3 fault domains.
An update domain is a logical grouping of servers spread across several server racks/fault domains, indicates groups of virtual machines and underlying physical hardware that can be rebooted at the same time. You can configure up to 20 update domains.

If you use one fault domain, you can use only one update domain.

Each virtual machine in your availability set is assigned an update domain and a fault domain by the underlying Azure platform.

Ref: https://learn.microsoft.com/en-us/azure/virtual-machines/availability-set-overview

2. Scale set

Azure Virtual Machine Scale Sets let you create and manage a group of load balanced VMs. The number of VM instances can automatically increase or decrease in response to demand or a defined schedule.

Key benefits:

Easy to create and manage multiple VMs
Provides high availability and application resiliency by distributing VMs across availability zones or fault domains
Allows your application to automatically scale as resource demand changes
Works at large-scale

2.1. Attach VM to a scale set

You can only attach Virtual Machines to a Virtual Machine Scale Set in Flexible orchestration mode. For more information, see Orchestration modes for Virtual Machine Scale Sets.

Ref: https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-attach-detach-vm

2.2. Orchestration mode

Virtual Machine Scale Sets with Uniform orchestration use a virtual machine profile or template to scale up to desired capacity. Azure VMs created by the Virtual Machine Scale Set with the uniform orchestration mode expose virtual machines as scale set instances, which has limited functionality compared to the regular IaaS VMs.

VMs created with the flexible orchestration mode expose actual virtual machine resources.

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